well tpdays CPU are designed for supporting a particular number of channel, so that's the reason why

there's actually 3 channel or "path" linking the ram to the controller on the motherboard which is on the CPU on 1366, and same design on 1155 but with 2 channel.

nothing stops you from using 2 sticks on 1366 running in dual channel, but 3 sticks are to be use to get full memory bandwidth and be in triple channel(real life performace drop is hardly noticiable)
but installing 3 stick in a 1155 mobo won't make you running triple channel

2 vs 3 channel performance is nothing to write home about, especially when a 1155 setup will yield much more bandwidth in a dual channel design than a 1366 setup can achieve in triple channel.

here's 2 picture showing the architecture of both platform. That's actuall the whole motherboard but we only need the top part for this. You can see on the right of the cpu the "path" or channel between the cpu(memory controller in on die with the cores) and the ram